The present invention relates to vacuum furnaces and is concerned primarily with the rapid quenching or cooling of work parts after the heat treatment thereof, and in this connection has application in both a continuous heat treating vacuum furnace of the type illustrated in applicant's prior U.S. Pat. No. 4,118,016 and in the batch-type of heat treating vacuum furnace, such as illustrated in applicant's prior U.S. Pat. No. 3,599,946. In both the continuous heat treating and the batch-type of vacuum heat treating furnaces as illustrated in the aforesaid U.S. patents, quenching of the heat treated work parts was accomplished by dumping the work parts directly into an oil vat immediately upon removal thereof from the heating chamber. Although vacuum quenching in oil has been normally successfully carried out heretofore in vacuum heat treating furnaces, in certain circumstances, and particularly if the temperature of the furnace was elevated above 2000.degree. F., rapid quenching in oil sometimes resulted in cracking of the parts. Therefore, careful attention had to be given to the operating characteristics of the vacuum operated furnace that provided for quenching of the heat treated parts in a liquid such as oil, since severe damage thereto could result if the quenching operation was not properly performed. Further, quenching in oil after a period of heat treatment at elevated temperatures tended to promote carburization of the work parts, which detracted from the fundamental purpose of heat treating the parts and rapidly quenching in a liquid such as oil.
A prior known procedure for quenching work parts after the heat treating thereof has been quenching in an atmosphere, such as nitrogen; although prior techniques for accomplishing this purpose have necessarily resulted in excessive quenching periods. Examples of vacuum furnaces known to applicant that have used an atmospheric quenching medium are illustrated in applicant's prior U.S. Pat. Nos. 3,257,492; 3,342,469 and 3,431,346. Since production requirements for heat treating work parts do require a relatively short cycle, the prior known method of quenching in an atmosphere is now unacceptable since the heat treatment cycle is unduly extended and furthermore is useful only in the batch-type of heat treating furnace. Further, in the prior known processes that utilized a cooling gas for quenching, the cooling gas was usually maintained at a subatmospheric pressure or at a pressure just slightly higher than atmosphere. Under these conditions, heat transfer was limited. As will be described hereinafter, increasing the pressure of the quenching gas, increases conductivity and density of gas, thereby accelerating heat transfer for promoting rapid cooling.